It is well-known that transmission lines have an inherent impedance and that devices, such as telephones, modems or signal processors, which are connected to transmission lines must match the transmission line impedance to reduce signal attenuation and reflection. In telephone systems there is a requirement that devices which are coupled to telephone lines must provide isolation between the device and the central office. Typically, prior art systems provided isolation by using a transformer between the telephone line and the device. The telephone line is usually connected to the primary side of the transformer and the device is on the secondary side. Signals from the transmission line and from the device are coupled across the transformer so that current does not flow directly from the device to the central office.
Instead of using a transformer to couple a device to a transmission line, some prior art systems use optically coupled circuits to provide the required isolation. This type of system is disclosed in U.S. Pat. Nos. 4,190,747 and 4,228,323, both to Feiner et al., the disclosure of which is hereby incorporated by reference. Optically coupled systems use photoemitters and photodetectors to couple the two sides of the circuit using light waves.
In the prior art transformer coupled and optically coupled devices there is still a need to provide impedance matching between the device and the transmission line. Typically, the impedance matching is accomplished by a matching circuit having discrete components that are selected to match an expected transmission line impedance to an expected coupling circuit impedance. These prior art circuits require the circuit designer to have a fairly accurate measurement of the transmission line impedance in order to properly match the device to the transmission line. If the device is connected to a different transmission line or if the transmission line impedance is not measured properly, then the matching circuit will be designed for the wrong impedance values and the circuit is likely to cause signal attenuation and reflection. A telephone line impedance is typically 600 xcexa9xc2x110%. The variation in impedance may be significant enough on different transmission lines to require modifying the matching circuit each time the device is attached is attached to a different transmission line.
Received and transmitted signals that pass through matching circuits are distorted by the transfer function of the matching circuit. If the matching circuit is adjusted for a new transmission line impedance, then the transfer function for the matching circuit will change. Therefore, each time a device is connected to a different transmission line, the matching circuit must be adjusted for the new transmission line impedance to achieve a perfect match and the transfer function must be recalculated for the modified matching circuit.
The present invention provides a system and method in which an optically coupled circuit provides an impedance match between a transmission line and another device, such as a telephone, modem or digital signal processor. The matching circuit is comprised of optical couplers which provide the required isolation between the transmission line and the device. In the prior art isolation circuits, the device on the secondary side cannot directly measure the impedance on the transmission line side of the isolation circuit because of the isolation across the transformer or the optical couplers.
In the present invention, a processor, such as a digital signal processor, monitors the signals that pass from the transmission line through the isolation and matching circuitry. A reference signal, such as a central office dial tone, is provided on the transmission line. The reference signal is detected by the processor after it passes through the matching circuit. It has been determined that harmonics of the reference signal are created when the matching circuit is not properly matched to the transmission line impedance. These harmonic signals appear at the output of the matching circuit. The processor detects these harmonic signals and adjusts the matching circuit impedance by varying the bias voltages of one or more optical couplers. The bias is adjusted in discrete steps and the processor monitors the amplitude of the harmonic of the reference signal at each step. The processor continues to adjust the optical coupler bias until the harmonic signals are driven to a minimum level. The processor also measures the noise level in the system. In the best case, the harmonics will be reduced to the noise level by the matching circuit.
The processor calibrates and balances the transmission and receive portions of the matching circuit after the harmonics are driven to a minimum level. Tide processor monitors the harmonic signal amplitude while balancing the circuit to ensure that the harmonics remain at a minimum level, preferably at or below the noise level.
After balancing the circuit, the processor transmits a signal into an input of the matching circuit and detects an echo at an output of the matching circuit. The processor then adjusts the gain in the transmission and receive paths to suppress echo signals from the receive output. Gain in the transmission and receive circuits can be adjusted by changing the resistance of a variable resistor or by varying the gain in a coder/decoder (codec) channel coupled between the matching circuit and the processor.
The matching circuit of the present invention also performs two-wire to four-wire conversion for signals that are received from a telephone line. The two-wire telephone line connection is converted to a four-wire connection for the processor. The four-wire connection has a dedicated transmit and receive lines, while the two-wire connection carries both transmitted and received signals. A codec is coupled to the four-wire output of the matching circuit to convert between the analog signals carried by the transmission line and the corresponding digital signals that are processed by the processor.
The transfer function of the optically coupled matching circuit can be determined after the matching circuit has been matched to the impedance of the transmission line and the transmission and receive circuit gains have been balanced and corrected for signal echo. Using the transfer function, received and transmitted signals can be modified to compensate for signal distortion caused by the matching circuit. The processor can determine the matching circuit transfer function by generating an impulse and transmitting the impulse into the matching circuit. The corresponding signal that appears at the receive output is sampled to determine an impulse response of the matching circuit.
The frequency domain transfer function for the matching circuit is derived by performing a Fourier transform on the impulse response in the processor. By inverting the frequency domain transfer function and performing an inverse Fourier transform on the frequency domain transfer function, the processor can generate a time domain transfer function that is the inverse of the matching circuit transfer function. Signals from the transmission line will distorted as they pass through the matching circuit. The processor can recover the original signal by convolving a distorted signal with the inverse transfer function so that the effects of the matching circuit transfer function are canceled. In a similar manner, signals transmitted by the processor can be predistorted in the processor using the inverse transfer function so that signals appearing on the two-wire side of the matching circuit are undistorted.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.